According to emulsion polymerization, the polymerization reaction generally occurs within micelles of a surfactant that are provided by a surfactant. The polymer produced in accordance with a progress of the polymerization is not soluble or hardly soluble in water, but due to an action of a surfactant, it is stably dispersed as fine particles in an aqueous medium without being separated or precipitated from the aqueous medium. Such polymer dispersion is referred to as latex.
If polymer latex is produced by emulsion polymerization, coarse particles with a size of 10 to 100 μm or so in which polymer particles are aggregated are produced in a large amount. Unlike a scale adhered to an inner wall of a reactor or a stirring wall, the coarse particles are included in latex in a dispersed state so that it is incorporated with latex to a step after the polymerization. Once the coarse particles are incorporated in a product, they are known to cause defects that are referred to as “protrusions” (hereinbelow, also referred to as a “fish eye”). For an optical application or an application requiring a decorative property, in particular, coarse particles with a size of several tens of μm or higher become a cause of defects in the product. As such, it is desired to suppress generation of coarse particles with a size of several tens of μm or higher that are produced during polymerization and also to remove the produced coarse particles.
The step for filtering polymer latex is to remove coarse particles that are generated during polymerization from latex or to remove foul substances that are incorporated in a raw polymer material or introduced from an outside during polymerization, and it is an essential step for reducing foreign substances. Hereinbelow, the coarse particles and foul substances are collectively referred to as “foreign substances”. When a filter medium mesh for filtering is decreased to reduce foreign substances, the filter medium is easily clogged by the foreign substances, in particular, the coarse particles. Accordingly, the lifetime of a filter medium is shortened so that the production speed inevitably needs to be slowed down and the filter medium replacement frequency is increased. Namely, decreasing the size of a filter medium mesh to reduce foreign substances has a problem that productivity for latex polymer is lowered and also it is no longer possible to continue stable production.
In other words, for producing polymer latex, reducing foreign substances and having simultaneously stable production and enhanced production efficiency are a difficult task to achieve. However, it is strongly desired to have both of them.
According to Patent Document 1, a method of mixing a monomer for polymerization with water and a surfactant and supplying them as an emulsion to a reactor is suggested as a method of suppressing scale during emulsion polymerization. In Patent Document 1, there is a description about an effect of reducing scale which does not pass through 60 mesh among scales adhered on an inner wall of a reactor and a stirring wing and scales contained in latex. However, no mention is made regarding reduction of coarse particles as a cause of protrusions and the filtering property of latex, and thus the problems still remain. In Patent Document 2, a method of reducing acetone insoluble particles with a size of at least 55 μm, which become a cause of fish eye in an acrylic film, is suggested. According to Patent Document 2, fish eyes having a size of 0.4 mm×0.5 mm or more in a film with a thickness of 50 μm were lowered by reducing acetone insoluble particles with a size of at least 55 μm. However, as there is no mention about fish eyes with a smaller size, it cannot be said that the method is sufficient by itself. Furthermore, in Patent Document 3, a method of removing an aggregate as a cause of fish eye by a specific filtering method is suggested. According to Patent Document 3, small fish eyes with a size of at least 25 μm are also reduced in an acrylic film with a thickness of 80 μm. However, as a standard for detecting fish eyes is loose, that is, having a transmission ratio of 50% or less, it is expected to have many fish eyes if the measurement is made with a transmission ratio of 75%. Furthermore, when a filtering mesh is 1 μm like Patent Document 3, the filtering medium is easily clogged so that production efficiency may be lowered. For further reducing fish eyes, it is essential to suppress generation of coarse particle as a cause of fish eye and reduce mesh size during filtration step. It is also essential to establish a method for producing latex with a good filtering property to reduce mesh size.